The present invention relates to preventive maintenance and repairing technology in, for example, a nuclear power plant and the like, and more specifically, to an underwater laser processing, such as inspecting, repairing or maintaining, method and an apparatus suitable for the improvement of residual stress, removal of a crack, removal of a CRUD (Chalk River Unidentified Deposite) and the like on a surface of a material constituting a metal structure.
Reactor components in a light-water reactor is composed of, for example, a material having sufficient corrosion resistance and strength against high temperature in the environment of high temperature and high pressure such as, for example, austenitic stainless steel, nickel base alloy or the like.
However, components in the reactor inner structure which are difficult to be replaced involve a problem that materials constituting the components are deteriorated because the materials are exposed to the environment of high temperature and high pressure for a long period of time as the plant is operated for a long time and further core materials constituting a core shroud and the like are subjected to the irradiation of neutron.
In particular, portions in the vicinity pf welding area of the reactor inner structure has a potential danger of the occurrence of stress corrosion cracking due to the sensitization of a material and residual tensile stress caused by welding heat input.
Recently, various material surface improving or modifying technologies have been developed as countermeasures in preventive maintenance so as to cope with the prolonged operation period of plants. As one of the countermeasures, there is examined a countermeasure for preventing stress corrosion cracking by positively changing residual stress on a surface of a material from tensile stress to compressive stress and, for example, there has been developed a material surface residual stress improving or modifying technology by, for example, a method of shot-peening, water-jet peening and the like.
The shot-peening is a technology for forming compressive residual stress on the material surface of a portion to be worked by plastical deformation of the material surface by kinetic energy of steel balls which is produced when the steel balls each having a diameter of about 0.3 mm-1.2 mm are accelerated making use of high-pressure air, high-pressure water or a centrifugal force, and then collide against the material surface.
The water-jet peening is a technology for forming compressive stress on the material surface by a water collisional action and a shock wave which is caused when cavitation is broken by injecting ultra-high pressure water of about 1000 atm. from the extreme end of a nozzle.
It has been proved that the shot-peening technology is effective to prevent stress corrosion cracking.
There is a possibility that portions in the vicinity of welds of the reactor internal structure and a structure in the environment of vibration are subjected to the occurrence and progress of stress corrosion cracking and cracks due to fatigue which are caused while a plant is operated for a long time and further to increased sensitiveness to breakage resulting from them. Thus, a countermeasure against them have been studied.
For example, when a crack is generated on a surface of a structure, there has been studied such a countermeasure that the cracked portion is removed by electric discharge machining so that stress concentration is relieved in a material of the structure and the progress of the crack is suppressed. The electric discharging is a processing method of generating electric discharge between an electrode and a material to be processed to thereby melt and blow off the material being processed.
It is known that a clad containing Fe, Cu, Ni etc. is deposited on a surface of the reactor internal structure. Since the CRUD traps radioactive materials, there is possibility that an amount of exposure to radioactivity of workers who carry out repair and preventive maintenance is increased by the presence of the CRUD. In particular, there is possibility that the CRUD deposited to a fuel element prevents the cooling effect of the fuel element effected by water, accelerates oxidation and further may destroy the fuel.
At present, the CRUD deposited on the surface of the reactor inner structure is removed at irregular intervals by a method of making use of high-pressure water flow such as water-jet peening and the like. In addition, there are also studied a method of tearing off the CRUD by the electric discharge processing or the irradiation of an ultrasonic wave or a method of removing the CRUD by weakening a shock wave generated by the steel balls by applying the shot-peening technology.
Incidentally, in the method of using the steel balls such as the shot-peening method and the like, it is necessary to arrange high-pressure piping for conveying the steel balls by means of the high-pressure water or high-pressure air, and when processing is carried out to a structure in a narrow portion such as an annular portion between an core shroud and a reactor pressure vessel or inner surfaces of fine pipes or tubes, it is difficult to perfectly achieve such processing. In addition, when the structure is worked in the atmosphere, this method is accompanied by a problem of the generation of dusts.
In the method of using high-pressure water such as the water-jet peening and the like, a load on peripheral equipment is increased by a reaction force resulting from jetted water and it is difficult to develop an automatic machine for carrying out precise processing in a narrow portion by a remote control manner.
The electric discharging technology has a problem that a material to be processed is greatly affected by heat and the ultrasonic method is difficult to be applied to a narrow portion where an ultrasonic wave is difficult to reach.
That is, any of the above processing methods has problems in simplicity, applicability to a narrow portion, effect on peripheral equipment and quality, and it is difficult to say that all of these conditions are satisfied. Further, it is very difficult to apply these methods to the three types of technologies, that is, the improvement of the residual stress, removal of cracks and removal of a CRUD relating to the preventive maintenance and repair.